Solvent-Free Polymorphism Control in a Covalent Mechanochemical Reaction

Four crystal modifications of a Schiff base derived from 2-hydroxy-1-naphthaldehyde and 2-aminobenzonitrile have been obtained by conventional solution-based methods. Three polymorphs out of four can be synthesized under solvent-free conditions. Herein we report simultaneous solvent-free covalent synthesis and polymorphism control of a single component system using mechanochemistry via neat grinding and seeding-assisted grinding, i.e. neat grinding in the presence of seed crystals. We describe the important role of seed crystals in directing the supramolecular organization of the product of covalent solvent-free reaction toward the intended polymorphic outcome. Single crystal X-ray analysis of the four crystal forms revealed conformational differences in molecules. Polymorphs display interesting and remarkably different molecular packing features via C-H center dot center dot center dot N and C-H center dot center dot center dot O interactions: 1D chains in form I, a 2D-network in forms III and IV, and a 3D-network in form II.